Petroleum is a term that includes a wide variety of liquid hydrocarbons . Many scientists also include natural gas in their definition of petroleum. The most familiar types of petroleum are tar, oil, and natural gas. Petroleum forms through the accumulation, burial, and transformation of organic material—such as the remains of plants and animals—by chemical reactions over long periods of time. After petroleum has been generated, it migrates upward through the earth, seeping out at the surface of the earth if it is not trapped below the surface. Petroleum accumulates when it migrates into a porous rock called a reservoir that has a non-porous seal or cap rock that prevents the oil from migrating farther. To fully understand how petroleum forms and accumulates requires considerable knowledge of geology , including sedimentary rocks , geological structures (faults and domes, for example), and forms of life that have been fossilized or transformed into petroleum throughout the earth's long history.
Tremendous petroleum reserves have been produced from areas all over the world. In the United States, the states of Alaska, California, Louisiana, Michigan, Oklahoma, Texas, and Wyoming are among the most important sources of petroleum. Other countries that produce great amounts of petroleum include Saudi Arabia, Iran, Iraq, Kuwait, Algeria, Libya, Nigeria, Indonesia, the former Soviet Union, Mexico, and Venezuela.
Petroleum products have been in use for many years. Primitive man might have used torches made from pieces of wood dipped in oil for lighting as early as 20,000 b.c. At around 5,000 b.c., the Chinese apparently found oil when they were digging underground. Widespread use of petroleum probably began in the Middle East by the Mesopotamians, perhaps by 3,000 b.c., and probably in other areas where oil seeps were visible at the surface of the earth. Exploration for petroleum in the United States began in 1853, when George Bissell, a lawyer, recognized the potential use of oil as a source of lamp fuel. Bissell also recognized that boring or drilling into the earth, as was done to recover salt, might provide access to greater supplies of petroleum than surface seeps. In 1857, Bissell hired Edwin Drake—often called "Colonel" Drake despite having worked as a railroad conductor—to begin drilling the first successful oil well. The well was drilled in 1859 in Titusville, Pennsylvania. Once the usefulness of oil as a fuel was widely recognized, exploration for oil increased. By 1885, oil was discovered in Sumatra, Indonesia. The famous "gusher" in the Spindletop field in eastern Texas was drilled in 1901. The discoveries of giant oil fields in the Middle East began in 1908 when the company now known as British Petroleum drilled a well in Persia (now Iran). During World Wars I and II, oil became a critical factor in the ability to successfully wage war.
Currently, petroleum is among our most important natural resources. We use gasoline, jet fuel, and diesel fuel to run cars, trucks, aircraft, ships, and other vehicles. Home heat sources include oil, natural gas, and electricity , which in many areas is generated by burning natural gas. Petroleum and petroleum-based chemicals are important in manufacturing plastic, wax, fertilizers, lubricants, and many other goods. Thus, petroleum is an important part of many human activities.
Petroleum, including liquid oil and natural gas, consists of substances known as hydrocarbons. Hydrocarbons, as their name suggests, comprise hydrogen and carbon , with small amounts of impurities such as nitrogen, oxygen , and sulfur. The molecules of hydrocarbons can be as simple as that of methane, which consists of a carbon atom surrounded by four hydrogen atoms, abbreviated as CH4. More complex hydrocarbons, such as naphthenes, include rings of carbon atoms (and attached hydrogen atoms) linked together. Differences in the number of hydrogen and carbon atoms in molecules as well as their molecular structure (carbon atoms arranged in a ring structure, chain, or tetrahedron, for example) produce numerous types of petroleum.
Different types of petroleum can be used in different ways. Jet fuel differs from the gasoline that automobiles consume, for example. Refineries separate different petroleum products by heating petroleum to the point that heavy hydrocarbon molecules separate from lighter hydrocarbons so that each product can be used for a specific purpose. Refining reduces the waste associated with using limited supplies of more expensive petroleum products in cases in which a cheaper, more plentiful type of petroleum would suffice. Thus, tar or asphalt, the dense, nearly solid hydrocarbons, can be used for road surfaces and roofing materials, waxy substances called paraffins can be used to make candles and other products, and less dense, liquid hydrocarbons can be used for engine fuels .
Petroleum is typically found beneath the surface of the earth in accumulations known as fields. Fields can contain oil, gas, tar, water , and other substances, but oil, gas, and water are the most common. In order for a field to form, there must be some sort of structure to trap the petroleum, a seal on the trap that prohibits leakage of the petroleum, and a reservoir rock that has adequate pore space, or void space, to hold the petroleum. To find these features together in an area in which petroleum has been generated by chemical reactions affecting organic remains requires many coincidences of timing of natural processes.
Petroleum generation occurs over long periods of time—millions of years. In order for petroleum generation to occur, organic matter such as dead plants or animals must accumulate in large quantities. The organic matter can be deposited along with sediments and later buried as more sediments accumulate on top. The sediments and organic material that accumulate are called source rock. After burial, chemical activity in the absence of oxygen allows the organic material in the source rock to change into petroleum without the organic matter simply rotting. A good petroleum source rock is a sedimentary rock such as shale or limestone that contains between 1% and 5% organic carbon. Rich source rocks occur in many environments, including lakes , deep areas of the seas and oceans , and swamps. The source rocks must be buried deep enough below the surface of the earth to heat up the organic material, but not so deep that the rocks metamorphose or that the organic material changes to graphite or materials other than hydrocarbons. Temperatures less than 302°F (150°C) are typical for petroleum generation.
Once a source rock generates and expels petroleum, the petroleum migrates from the source rock to a rock that can store the petroleum. A rock capable of storing petroleum in its pore spaces, the void spaces between the grains of sediment in a rock, is known as a reservoir rock. Rocks that have sufficient pore space through which petroleum can move include sandstone , limestone, and rocks that have many fractures. A good reservoir rock might have pore space that exceeds 30% of the rock volume. Poor quality reservoir rocks have less than 10% void space capable of storing petroleum. Rocks that lack pore space tend to lack permeability , the property of rock that allows fluid to pass through the pore spaces of the rock. With very few pores, it is not likely that the pores are connected and less likely that fluid will flow through the rock than in a rock with larger or more abundant pore spaces. Highly porous rocks tend to have better permeability because the greater number of pores and larger pore sizes tend to allow fluids to move through the reservoir more easily. The property of permeability is critical to producing petroleum: if fluids can not migrate through a reservoir rock to a petroleum production
well, the well will not produce much petroleum and the money spent to drill the well has been wasted.
In order for a reservoir to contain petroleum, the reservoir must be shaped and sealed like a container. Good petroleum reservoirs are sealed by a less porous and permeable rock known as a seal or cap rock. The seal prevents the petroleum from migrating further. Rocks like shale and salt provide excellent seals for reservoir rocks because they do not allow fluids to pass through them easily. Seal-forming rocks tend to be made of small particles of sediment that fit closely together so that pore spaces are small and poorly connected. The permeability of a seal must be virtually zero in order to retain petroleum in a reservoir rock for millions to hundreds of millions of years, the time span between formation of petroleum to the discovery and production of many petroleum fields. Likewise, the seal must not be subject to forces within the earth that might cause fractures or other breaks in the seal to form.
Reservoir rocks and seals work together to form a trap for petroleum. Typical traps for petroleum include hills shaped similar to upside-down bowls below the surface of the earth, known as anticlines, or traps formed by faults. Abrupt changes in rock type can form good traps, such as sandstone deposits next to shale deposits, especially if a sand deposit is encased in a rock that is sufficiently rich in organic matter to act as a petroleum source and endowed with the properties of a good seal.
An important aspect of the formation of petroleum accumulations is timing. The reservoir must have been deposited prior to petroleum migrating from the source rock to the reservoir rock. The seal and trap must have been developed prior to petroleum accumulating in the reservoir, or else the petroleum would have migrated farther. The source rock must have been exposed to the appropriate temperature and pressure conditions over long periods of time to change the organic matter to petroleum. The necessary coincidence of several conditions is difficult to achieve in nature.
Petroleum exploration and production activities are performed primarily by geologists, geophysicists, and engineers. Geologists look for areas of the earth where sediments accumulate. They then examine the area of interest more closely to determine whether or not source rocks and reservoir rocks exist there. They examine the rocks at the surface of the earth and information from wells drilled in the area. Geologists also examine satellite images of large or remote areas to evaluate the rocks more quickly.
Geophysicists examine seismic data, data derived from recording waves of energy introduced into the rock layers of the earth through dynamite explosions or other means, to determine the shape of the rock layers beneath the surface and whether or not traps such as faults or anticlines exist.
Once the geologist or geophysicist has gathered evidence of potential for a petroleum accumulation, called a prospect, an engineer assists in determining how to drill a well or multiple wells to assess the prospect. Drilling a well to explore for petroleum can cost as little as $100,000 and as much as $30,000,000 or more, depending on how deep the well must be drilled, what types of rocks are present, and how remote the well location is. Thus, the scientists must evaluate how much the well might cost, how big the prospect might be, and how likely the scientific predictions are to be correct. In general, approximately 15% of exploration wells are successful.
Once a successful exploration well has been drilled, the oil and/or gas flow are pumped to the surface of the earth through the well. At the surface, the petroleum either moves through a pipeline or is stored in a tank or on a ship until it can be sold.
Estimates of the amount of recoverable oil and natural gas in the United States are 113 billion barrels of oil and 1,074 trillion cubic feet of natural gas. Worldwide estimates of recoverable oil and natural gas are 1 trillion barrels of oil and 5 quadrillion cubic feet of natural gas. These worldwide reserves are expected to supply 45 years of fuel at current production rates with expected increases in demand. However, such estimates do not take into account reserves added through new discoveries or through the development of new technology that would allow more oil and natural gas to be recovered from existing oil and natural gas fields.
Daily consumption of oil in the United States exceeds 17 million barrels of oil per day, of which approximately 7 million barrels are in the form of gasoline for vehicles. Over half the petroleum consumed in the United States is imported from other countries. (Assuming oil costs $20 per barrel and 8.5 million barrels per day are imported, over one billion dollars per week are spent on oil imports). While the United States has tremendous reserves of petroleum, the undiscovered fields that remain tend to be smaller than the fields currently producing petroleum outside of the United States. Thus, less expensive foreign reserves are imported to the United States. When foreign petroleum increases in price, more exploration occurs in the United States as it becomes more profitable to drill wells in order to exploit smaller reservoirs.
Current research in petroleum includes many different activities. Within companies that explore for and produce petroleum, scientists and engineers try to determine where they should explore for petroleum, how they might recover more petroleum from a given field, and what types of tools can be lowered into wells in order to enhance our understanding of whether or not that individual well might have penetrated an oil or gas field. They also examine fundamental aspects of how the earth behaves, such as how rocks form and what forms of life have existed at various times in the earth's history. The United States Geological Survey continues to evaluate petroleum reserves and new technology to produce oil and gas. The federal government operates several facilities called Strategic Petroleum Reserves that store large quantities of petroleum for use in times of supply crisis.
Petroleum exploration specialists are using a type of geophysical data known as three-dimensional seismic data to study the structures and rock types below the surface of the earth in order to determine where exploration wells might successfully produce petroleum. Geochemists are assessing the results of studies of the chemistry of the surface of the earth and whether or not these results can improve the predictions of scientists prior to drilling expensive exploratory wells.
Significant recent discoveries of petroleum have been made in many areas of the world: Algeria, Brazil, China, Egypt, Indonesia, the Ivory Coast, Malaysia, Papua New Guinea, Thailand, the United Kingdom, and Vietnam, among others. In the United States, the Gulf of Mexico , Gulf Coast states, California, and Alaska continue to attract the interest of explorers.
See also Fossils and fossilization; Fuels and fuel chemistry; Geochemistry; Petroleum detection; Petroleum, economic uses of; Petroleum extraction; Petroleum, history of exploration; Sedimentation; Syncline and anticline